In a stepper such as an X-ray exposure apparatus, it is necessary to adjust a mask and a wafer in place with a high accuracy.
In a stepper, a resist film, transparent film and the like are provided on a surface of a wafer. Since these films are not uniformly formed, a reflection intensity of an alignment mark on the wafer varies sinusoidally due to a standing wave effect based on optical interferences and a relative intensity of signal rays to detect the alignment mark varies largely, resulting in that an accuracy of alignment is remarkably reduced.
To eliminate such disadvantages and improve the accuracy of alignment a variety of proposals have been offered. For example, the scientific lecture report of the 48th Meeting of Applied Physics Society (the second separate volume 18a-F-6, P.426, Autumn, 1987) shows that when two wavelength rays or two inclination illuminations are applied in detection an alignment mark on a wafer in a bright visual field, the unsymmetry of a detected pattern with respect to unevenness in resist coating is reduced to improve an accuracy of detecting a mark.
In a stepper manufactured by CENSOR company, two rays of wavelengths of 547 nm (e ray) and 578 nm (d ray) are employed as an alignment ray and when a sufficient signal ray is not obtained by one of the rays, a changeover to the other wavelength ray is made to conduct an alignment operation. According to experiments conducted by CENSOR, a relation between alignment signals by the two wavelength rays and a film thickness is shown in FIG. 15 (The practical hand-book of apparatus for manufacturing semiconductors, Science Forum Co. Ltd, PP. 224 to 225, Dec. 25. 1984).
Furthermore, LFF Process Control Division reports sinusoidal variation in reflection intensity due to interferences caused by each of two rays of wavelengths of 405 nm (h ray) and 436 nm (g ray) and variation in reflection intensity due to interferences caused by plural wavelength rays of g ray+h ray and g ray+h ray+i ray (LEE Corporation Process Control Division (60), Trapelo Road Walhm, Mass. 02154, (617) 890 to 2000).
This gives, as shown in FIG. 16, that with two wavelength rays of g+e an interference intensity of reflected rays is greatly improved at 10,000.+-.200 .ANG..
An example of contemplating improvement in terms of process has been indicated in the scientific lecture report of the 48th Meeting of Applied Physics Society (the second separate volume 18a-F-9, P.427, Autumn, 1987). According to the report, variations in interference intensity and unsymmetry of a mark are reduced by making thickness of a resist film on a wafer uniform to improve an accuracy of alignment.